The present invention relates to methods for writing patterns onto spinning recording disks, and to data storage devices for executing the same. More particularly, the invention concerns the servo correction process conducted during pattern writing onto a recording disk.
Data storage devices using various types of media such as optical disks and magnetic tapes are known in the art. Among them, hard disk drives (hereinafter referred to as HDDs) have become popular as storage devices for computers to such an extent that they are one type of the storage devices indispensable for today's computers. Further, not limited to computer systems, HDDs are expanding more and more in application because of their excellent characteristics. For example, HDDs are used for movies recording/reproducing devices, car navigation systems, cellular phones, and removable memories for use in digital cameras.
The magnetic disks used in HDDs each have a plurality of concentrically formed tracks, and servo data and user data are stored into each of the tracks. A magnetic head formed of a thin-film element can write or read data by accessing a desired region (address) in accordance with servo data. The signals that the magnetic head has read out from the magnetic disk during a data readout process are subjected to waveform shaping, decoding, and other required signal processing, by a signal-processing circuit, and then transmitted to a host. After undergoing required processing by the signal-processing circuit similarly to the above, transfer data from the host is written onto the magnetic disk.
As described above, each track includes a data region into which data is stored, and a servo pattern region into which servo data is stored. Each servo pattern is composed of a cylinder ID, a sector number, a burst pattern, and other data. The cylinder ID denotes the address of a track, and the servo ID denotes the internal sector address within the track. The burst pattern contains relative position information of the magnetic head with respect to the track.
Servo patterns made up of plural sectors are spaced in a circumferential direction on each track, and the servo patterns of each sector, across all tracks, are the same in circumferential position (i.e., in phase). Data reading out from or writing onto the magnetic disk is executed during rotation of the disk while it is confirming the position of the magnetic head in accordance with servo data.
Servo patterns are written onto magnetic disks in factories before HDDs are shipped as products. Conventional writing of servo patterns typically uses a servo writer as an external device. An HDD with its top cover removed is set in the servo writer, then the servo writer uses a positioner (external positioning mechanism) to precisely position the internal head of the HDD, and the servo patterns that have been generated by a servo pattern generator are written onto the magnetic disk.
Currently, a servo data writing process (Servo Track Write: STW) occupies the substantial portion in the manufacturing costs of HDDs. In recent years, in particular, HDDs have been exposed to intensified competition for higher capacity, and along with this tendency, TPI (Tracks Per Inch) is increasing. As TPI increases, the number of tracks increases and track width decreases. These, in turn, increase an STW time, enhance the precision of servo writers, and result in increased STW costs. Cost reduction of the servo writers, reduction in STW time, and others are being accelerated to reduce those costs.
For example, SSW (Self-Servo Write) uses only the mechanical assembly of an HDD unit to control the internal spindle motor and voice coil motor of the HDD from an external circuit, and uses the external circuit to write servo patterns. This reduces the costs of the servo writer.
An SSW method is known that utilizes the fact that the read element and write element of a head element differ from each other in radial position (this difference is called the read/write offset). In this method, positioning is conducted while the read element reads the servo patterns that have already written on the inside or outside diametral portions of the disk, and the write element writes new servo patterns onto desired tracks distant by the read/write offset.
In self-propagation of the servo patterns thus written onto new tracks, radial accuracy of the servo patterns is expected to be basically inherited to the new servo patterns written without degradation. Because of the factors that cause various errors, however, the accuracy of those patterns is deteriorated by the propagation. A decrease in positioning accuracy causes burst patterns to deviate from where they originally should be, and these deviations are inherited during the next propagation. This process also depends on the coefficient of the servo loop used to make the head element section follow a track. The deterioration of accuracy is of a complex mechanism involving various factors in this way.
In order to prevent the deterioration of track-following accuracy due to such propagation, the following SSW method is proposed in Patent Reference 1 (Japanese Patent Laid-open No. 8-212733). For example, consider a case in which current track TO undergoes track following to write new track TN. A position error signal (PES) by track following during the writing of track TO is stored. A Fourier transform is conducted for the PES, and each resultant frequency components is multiplied by a modification coefficient. This modification coefficient is provided for only high-gain components among all closed-loop response frequency components of the positioning servo of the head element. These high-gain components are usually limited only to several components of low frequencies. Next, an inverse Fourier transform is conducted for each of these components, thus correcting target data associated with each sector during track following to track TO to write patterns at track TN. This suppresses radial propagation errors.